A High Speed Aerodynamic Paradigm

By John Gilkison and Philip Knox

Aerodynamic streamlining was the craze in the 1930s. Now it could become the paradigm by which we design future motor vehicles.

In our recent blog in EV World entitled "100 MPH and 32 MPG" we introduced the concept of a high speed lane on the nation's rural interstate highway system where a medium sized car could do 100 MPH and still get the same mileage cars now get at 75 MPH. We limited our discussion to a generic 1996 Honda Accord to show what was possible with improved aerodynamic body forms with matched gearing, tires, and the safety equipment needed to do 100 MPH.

Cars

Since our nation's interstate highway system is designed for such high speeds we felt that doing justice to such a high speed paradigm shift discussion we should include most of the other classes of vehicles.

This high speed aerodynamic paradigm shift discussion deserved a full-fledged article with a few pictures and charts to do the subject proper justice. The drawing of a CD 0.13 car profile by Phil Knox illustrates what we mean.

This drawing by Phil Knox illustrates the side profile of a vehicle needed to reach a CD of 0.13. The aft body and radius of curvature is of critical importance. Stylist would have some liberty with the fore body design but they cannot change this aft body design which is the key to minimum drag. This aft body shape can be made deployable or stow able for highway / city driving.NOTE: All illustrations provided by the authors.

We are aware that some will wonder if we have not wandered off into some kind of fantasy unicorn garden proposing such a radical changes in our long accepted mode of highway travel, but we feel the discussion is long overdue and that the science is on our side.

All of our high speed travel proposals are based on well established science dating back to the 1920's and much of what we have to propose is known science inside the automotive industry. There may be other reasons why such aerodynamic body forms as we are proposing are not used, but they are not because they would not work as we envision here.

One reason may be that the need for such exotic body forms has not been seen as necessary. Fuel is still relatively inexpensive, and the speed limits on most interstate highways are still only 75 MPH. More importantly elongated body forms such as we portray here, are to put it mildly, inconvenient to operate in the city or even to park in your garage.

There is a way out of this conundrum which needs to be given serious consideration. Boat tail forms to vehicle platforms need not be fixed but can be made to be deployable, a concept which dates back to the 1930's. They could be inflatable or rigid deployable forms that would only be used on the highway. Also they could be added as needed in the form of a boat tail trailer that would be towed. Finally there are swing arm mounted hitch mounted travel trunks that could be vehicle specific in shape forming a boat tail when mounted for highway travel. I have seen these large cargo swing arm mounted carryalls mounted on motor homes. This form of boat tail with gap fillers would work well in SUV's and light trucks.

This Hot Rod Magazine graph of the relationship between the needed horsepower to maintain speed at various coefficients of drag illustrates why low drag forms are important in fuel efficiency

Our stated goal is to achieve a CD of 0.130 making vehicles capable of returning the same fuel mileage they currently get at 75 MPH and a new higher speed of 100 MPH for cars and light trucks, and 90 MPH for larger vehicles like intercity buses, semi trucks, motor homes and other RV's.

Vehicles without the proper fuel saving shape would still be able to use the rural interstate highways at the currently posted speeds using the far right lane.

Trucks and Buses

What about full sized cars, trucks, intercity passenger buses, and tractor trailer trucks for moving freight? Vehicles with larger frontal areas, more wetted surface area, and mass would have to be treated differently. All things being equal, fuel savings is a comparative art at best. It is going to take a certain amount of energy to move a given shape and tire design through the air at speed based on the size and mass of the vehicle and you cannot expect all vehicle classes to return the same fuel burn rate accomplishing this work. Your comparison vehicle for demonstrating improvement is the base vehicle itself.

Trucks and buses as we know them now are simply too tall increasing drag excessively. In intercity passenger buses the floor of the passenger compartment is above the luggage storage compartments giving them this excessively high profile. Tractor trailer rigs have matured in a design paradigm dictating four foot high dock heights.

We admit that there are practical reasons for some of this evolution, but these considerations can be over ridden using a new paradigm where higher speed without a fuel consumption penalty is the new norm. Because the current rules usually mandate a 10 MPH differential between trucks and cars allowing cars to go faster, we would like to see that speed differential maintained. Our recommendation is 100 MPH for cars and 90 MPH for trucks and buses. The ability for regular traffic to pass larger vehicles like this is a good idea, and 90 MPH would decrease travel times for freight haulers and passenger buses over what they see today significantly.

Our aerodynamic intercity passenger bus as shown without a standard high floor is about three foot lower then a stock bus today. Towing a boat tail luggage carrying trailer with a flexible gap filler lowers the CD to 0.13. With proper gearing at 90 MPH it would return better fuel mileage than buses do now. The weight of the bus would be reduced, but additional weight is reintroduced towing the trailer, so we treated this new form as if there was no weight difference.

Our aerodynamic boat tail luggage trailer towing intercity bus shown at the bottom for a height comparison with a stock bus profile form currently used today.

Such a low-drag lower-height form would have much greater stability with a lower center of gravity. The risk of a fatal rollover accident would be greatly reduced. The interior height of the passenger compartment would remain the same.

Our Don Burr truck pictured below is not the final iteration of this low-drag form. A full tractor and trailer redesign would be needed to lower the truck from its current height reducing the wake area significantly. The improved transit time for cargo transport at 90 MPH would more than make up for lost cargo capacity. We think an aerodynamic high speed medium sized truck of no more 55,000 lbs would be of interest to businesses like Fed Ex, UPS, and other carriers.

Photo of a Don Burr truck outline as designed for the European market. This truck is still too high and lacks the proper length to form a boat tail to get the 0.13 CD shape needed for our high speed 90 MPH rural interstate highway paradigm.

Both the high speed intercity bus and tractor trailer scenario require higher speed rated tires than currently available for trucks and buses today. Fuel mileage would increase proportionally at the slower speeds of 75 MPH, 65 MPH, and 55 MPH as it did in the Honda Accord scenario in our blog '100 MPH and 32 MPG'.

Aerodynamic redesigns like this would need to be applied to RV Motor Homes, trailers, 5th wheel trailers, and other vehicles used in long distance interstate travel for them to be able to do 90 MPH without fuel penalties. The far right lane would be need maintained for slower traffic as it currently is in many locations.

For this new high speed paradigm to work as we envision it, a third lane would have to be built on rural interstates as outlined in our blog. The second middle lane would be reserved for 90 MPH traffic, and the far left lane would be for 100 MPH cars.

There would be no impact on the nation's vehicular fleet average composite fuel mileage which for cars is now 17 MPG. By necessity, all vehicles would have to operate at the lower speeds of 75 MPH, 65 MPH, and 55 MPH when passing major cities. An additional five cent or more fuel tax would raise $10G per as needed for highway infrastructure.

Once passenger cars reached their destination city an automated system would stow the inflatable boat tail or rigid panels back into the trunk of the vehicle. There would be an override switch on the dash labeled City/Highway and when in the City position the boat tail would not deploy at all. When in the Highway position the system would be enabled to deploy at the speed of 45 MPH.

If a vehicle was towing a boat tail trailer or a swing arm boat tail cargo carrier, the trailer or appliance would have to be removed at the destination if desired. Most people would probably opt for a deployable system but there are applications where a trailer or swing arm appliance would be more desirable. Light trucks and SUV's come to mind.

Rear facing cameras would be a necessity for applications where the rear view of the vehicle is obstructed. Camera systems also would be useful for replacing side mirrors and reducing residual drag induced by these appliances.

We think one of the reasons aerodynamic body forms have not been pursued more aggressively is the low top speed limit on our nation's highways. Increasing this speed limit with an increase in overall MPG's would make such low-drag forms very desirable.

Conclusions

How is the country to pay for all this? A five cent increase in the federal fuel tax on gasoline and diesel would raise approximately ten billion dollars a year. The fuel tax could be adjusted as needed. This revenue stream would enable the federal government to add a third lane to the nation's rural interstate highways over time. As we discussed in our blog, we would start work on the sunbelt highways first then work northward.

Once vehicles and the infrastructure become available individuals and commercial entities would be able to use the highways as the proper safety and electronic infrastructure became operable. These newer high speeds would accelerate commerce and travel times. We think it would render most airline travel less than 600 miles moot given the travel times to and from and the delays experienced at airports.

Imagine the amount of profits to be made in the sales of new vehicles to meet this desire for high speed travel. A secondary market in peripheral devices and controls related to high speed travel would develop. None of this would entail an increase in the nation's motor fuel consumption or production of greenhouse gases.

In point of fact, travel at slower speeds would increase fuel mileage dramatically with a net result of a possible decrease in the nation's fuel consumption rate. This is because of the necessity of slower speeds bypassing or entering urban areas.

We view the nation's trucking fleet as the most problematic issue in this high speed paradigm shift. Trucks as currently designed use a frame under the trailer floor which would have to be eliminated to lower the profile of the vehicle. Curtain walls in trailers would have to be exchanged for structural load bearing walls as seen in aircraft construction. The new lower profile trailers would lose some cargo carrying capacity.

Other problems remain as in the current standard dock height of four feet. Federal mandates as to total overall length and state laws about axle placement would have to be modified also. This is why we suggested that there may be a few carriers who have a need for higher speed deliveries that would help get the concept introduced into the market place. Other carriers would come along as they desired after they could make changes to their in house infrastructure.

The high speed tire rating issue would have to be solved from the beginning and that is up to the tire manufacturers. Since they currently make specialty tires for the airlines and even NASCAR we see no reason why such a 90 MPH speed rated truck or bus tire could not be made if the manufactures decided to do it.

Trucking beyond a 600 mile radius is problematic given the cost savings to the nation and environment of using piggy back railroad shipping of goods. If mandated, gradually over time we could reduce the number of trucks on the highways and make up for lost revenue with a minimal increase in the fuel tax.

Railroad aerodynamics is another issue that could be addressed. One of the best ways to improve the nation's fuel consumption relative to railroads is to convert the railroad system to overhead electric lines and to get rid of diesel engines for powering trains.

This way trains could even be powered by a mix of wind and solar generated electricity in some areas. We have not even discussed hybrid electric drives, battery only drives in cars, or micro-turbine powered hybrid drives in trucks. What is possible is only limited by our vision and the settled knowledge of aerodynamic science.

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